Magnitude squared coherence
Cxy = mscohere(x,y)
Cxy = mscohere(x,y,window)
Cxy = mscohere(x,y,window,noverlap)
[Cxy,W] = mscohere(x,y,window,noverlap,nfft)
[Cxy,F] = mscohere(x,y,window,noverlap,nfft,fs)
[Cxy,F] = mscohere(x,y,window,noverlap,f,fs)
[...] = mscohere(x,y,...,'twosided')
mscohere(...)
Cxy = mscohere(x,y)
finds
the magnitude squared coherence estimate, Cxy
,
of the input signals, x
and y
,
using Welch's averaged modified periodogram method.
The input signals may be either vectors or two-dimensional matrices.
If both are vectors, they must have the same length. If both are matrices,
they must have the same size, and mscohere
operates
columnwise: Cxy(:,n) = mscohere(x(:,n),y(:,n))
.
If one is a matrix and the other is a vector, then the vector is converted
to a column vector and internally expanded so both inputs have the
same number of columns.
For real x
and y
, mscohere
returns
a one-sided coherence estimate. For complex x
or y
,
it returns a two-sided estimate.
mscohere
uses the following default values:
Parameter | Description | Default Value |
---|---|---|
| FFT length which determines the frequencies at which the coherence is estimated For real If | Maximum of 256 or the next power of 2 greater than the
length of each section of |
| Sampling frequency | 1 |
| Windowing function and number of samples to use for each section | Periodic Hamming window of sufficient length to obtain
eight equal sections of x and |
| Number of samples by which the sections overlap | Value to obtain 50% overlap |
Note
You can use the empty matrix, |
Cxy = mscohere(x,y,window)
specifies
a windowing function, divides x
and y
into
equal overlapping sections of the specified window length, and windows
each section using the specified window function. If you supply a
scalar for window
, then Cxy
uses
a Hamming window of that length.
Cxy = mscohere(x,y,window,noverlap)
overlaps
the sections of x
by noverlap
samples. noverlap
must
be an integer smaller than the length of window
.
[Cxy,W] = mscohere(x,y,window,noverlap,nfft)
uses
the specified FFT length nfft
to calculate the
coherence estimate. It also returns W
, which is
the vector of normalized frequencies (in rad/sample) at which the
coherence is estimated. For real x
and y
, Cxy
length
is (nfft
/2 + 1)
if nfft
is even; if nfft
is
odd, the length is (nfft
+ 1)/2. For complex x
or y
,
the length of Cxy
is nfft
. For
real signals, the range of W
is [0, π]
when nfft
is even and [0, π)
when nfft
is odd. For complex signals, the range
of W
is [0, 2π).
[Cxy,F] = mscohere(x,y,window,noverlap,nfft,fs)
returns Cxy
as
a function of frequency and a vector F
of frequencies
at which the coherence is estimated. fs
is the
sampling frequency in Hz. For real signals, the range of F
is
[0, fs
/2] when nfft
is
even and [0, fs
/2) when nfft
is
odd. For complex signals, the range of F
is [0, fs
).
[Cxy,F] = mscohere(x,y,window,noverlap,f,fs)
computes
the coherence estimate at the frequencies f
. f
is
a vector containing two or more elements.
[...] = mscohere(x,y,...,'twosided')
returns
a coherence estimate with frequencies that range over the whole Nyquist
interval. Specifying 'onesided'
uses half the Nyquist
interval.
mscohere(...)
plots the
magnitude squared coherence versus frequency in the current figure
window.
Note If you estimate the magnitude squared coherence with a single window, or section, the value is identically 1 for all frequencies [1]. You must use at least two sections. |
[1] Stoica, Petre, and Randolph Moses. Spectral Analysis of Signals. Upper Saddle River, NJ: Prentice Hall, 2005.
[2] Kay, Steven M. Modern Spectral Estimation. Englewood Cliffs, NJ: Prentice-Hall, 1988.
[3] Rabiner, Lawrence R., and Bernard Gold. Theory and Application of Digital Signal Processing. Englewood Cliffs, NJ: Prentice-Hall, 1975.
[4] Welch, Peter D. "The Use of Fast Fourier Transform for the Estimation of Power Spectra: A Method Based on Time Averaging Over Short, Modified Periodograms." IEEE^{®} Transactions on Audio and Electroacoustics. Vol. AU-15, 1967, pp. 70–73.
cpsd
| periodogram
| pwelch
| spectrum
| tfestimate